How to send GET request with body?

[bycolour2]

I am trying to send a GET request with a body, but I receive the error: "Request with GET/HEAD method cannot have body."

I know that sending a GET request with a body is unusual, but I need to do it because of a legacy requirement.
What should I do?

[AlakhiarovSalekh]

The error is expected because the Fetch API does not allow a request body for GET or HEAD. Since RESTDataSource ultimately relies on Fetch-compatible request semantics, this cannot be enabled just by changing a RESTDataSource option.
If you control the API, the best solution is to move the payload into query parameters or change the endpoint to POST.
If the legacy upstream absolutely requires a GET request with a body, I would put a small adapter/proxy in front of it: let RESTDataSource make a normal request to the adapter, then have the adapter call the legacy service using a lower-level HTTP client that supports that non-standard request.
I would avoid relying directly on GET-with-body even if a low-level client permits it, because proxies, caches, gateways, and other HTTP infrastructure may ignore or reject the body.